
Immediate relatives are only parents, spouses, and children (unmarried and under 21) of US Citizens.
As you are a green card holder, you can only file for Spouses and children in category 2A or unmarried son/daughter (over 21) in category 2B. How long it would take would depend on priority dates and country of nationality.
See current priority dates below. Any other route would depend on their and whether they can find an employer, etc.
http://travel.state.gov/visa/bulletin/bulletin_1360.html